I agree with the other posters. After this length of time she should be over the initial capital drain of setting up buying equipemnt stocks etc and should now have an idea of how the business is doing. We aren't talking about helping her out in the early days to get a bit of cash behind her.
Basically if she has to get 3 people to work for free to cover her costs she's in trouble, even if two of which only work weekends and after school/ evenings. However thinking about it that's not just a few hours a week and weekends are probably her busiest times. Her business isn't viable and you need to ask yourself and her what would happen if she had to get paid staff.
As yourself if anyone else would work those hours regularly for no pay ? Would your dh do regular work that ate into his leisure family and personal time for no pay? This has gone from helping out to being taken for granted.
I can't see your dhs Pov about her being much of a role model - dd isn't much of an entrepreneur if she can't keep her business afloat without unpaid labour and is setting herself up for a huge problem when they both have enough and stop.
A poster above makes a good point about studying - this must be impacting on their studies. Your 15yo must definately be studying for GCSEs and won't be able to put in this amount of time much longer and the 18yo will either be studying for a levels or will want to have a paying job. What does your dh think about that?
If it was your family's business and the income was keeping a roof over their heads then I think he would have a point about everyone all pulling together and doing their bit. But that's not the case, they really aren't getting much out of it at all. They are giving up their free time, time they could be going hobbies, spending this with friends studying or even getting a proper paying part time job.
If it wasn't for the impact on studying I might be quiet and let the siblings work it out for themselves but make it clear they can quit if they wanted to.
But I think the impact on studies is too important so have a word with your oldest if you think she's making unreasonable demands on their time.
What would your prefer a daughter with a failed business and 2 others with good grades or a daughter with a failed business and 2 others with poor grades with the associated impact on their futures.